The present invention relates to digital television systems, such as the digital high-definition television (HDTV) system used for terrestrial broadcasting in the United States of America in accordance with the Advanced Television Sub-Committee (ATSC) standard, and more particularly, to digital television (DTV) signal receivers with filter circuitry for suppressing co-channel interference from analog television signals, such as those conforming to the National Television Systems Committee (NTSC) standard.
Digital television (DTV) signal radio receivers convert digital television signals to baseband symbols and perform symbol decoding using data-slicing procedures. DTV signal radio receivers are found in complete digital television sets provided with viewing screens, and the inventor envisions that DTV signal radio receivers will be found in digital tape recorders. In digital tape recorders it is desirable to remove co-channel interfering analog TV signal before tape recording, so that the time-base stability in the co-channel interfering analog TV signal is good enough that comb filtering can be employed that involves differential delay of DTV signals that extends over several horizontal scan lines. Symbol decoding is done after The comb filtering to suppress co-channel interfering analog TV signal is followed by symbol decoding, so that the recoding effects of the comb filtering can be compensated for during the symbol decoding procedures. The decoded data can then be recoded in accordance with a symbol coding suitable to digital tape recording. The symbol coding can, for example, comprise interleaved non-return to-zero, invert-on-ONEs (I-NRZI) modulation.
A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Subcommittee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television Subcommittee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel.
The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal: The number of data symbols per horizontal scan line period of an NTSC analog TV signal is 684, since 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 megasymbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV display format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV display format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV display formats other than HDTV display formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. The data fields may be considered to be consecutively numbered modulo-2, with each odd-numbered data field and the succeeding even-numbered data field forming a data frame. The frame rate is 20.66 frames per second. Each data segment is of 77.3 microseconds duration. So, with the symbol rate being 10.76 MHz there are 832 symbols per data segment. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, xe2x88x92S, xe2x88x92S and +S. The value +S is one level below the maximum positive data excursion, and the value xe2x88x92S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or xe2x80x9cPN-sequencexe2x80x9d) followed by three 63-sample PN sequences. The middle ones of the 63-sample PN sequences in the field synchronization codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one""s complementary respective to each other.
The data within data lines are trellis coded using twelve interleaved trellis codes, each a ⅔ rate trellis code with one uncoded bit. Prior to being trellis coded, the data are subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The trellis coding coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission, which transmissions are made without symbol preceding separate from the trellis coding procedure. The trellis coding coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without preceding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of xe2x88x927, xe2x88x925, xe2x88x923, xe2x88x921, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized vale of 1.25. The normalized value of +S is +5, and the normalized value of xe2x88x92S is xe2x88x925.
The current ATSC DTV standard presumes that the suppression of co-channel interfering analog TV signal will be carried out by the DTV signal receiver in the trellis decoding process, after the data-slicing procedures associated with symbol decoding. However, co-channel interfering analog TV signal undesirably introduces errors into the data-slicing processes, which places more burden on the error-correction decoding procedures, trellis decoding and Reed-Solomon decoding. These errors reduce the distance the DTV signal receiver can be from the DTV transmitter and still be assured satisfactory reception. So, providing for the suppression of co-channel interfering analog TV signal by introducing comb filtering before data-slicing is desirable, particularly for suppressing video content in the analog TV signal, if the DTV signal receiver is to be remote from the DTV transmitter. The current ATSC DTV standard does not provide for preceding of all data at the DTV transmitter, so as to compensate for the use of comb filtering before data-slicing within the DTV receiver.
Insofar as the co-channel interference from analog television signals is concerned, it enters the system channel after the DTV transmitter and before the DTV receiver. The use or non-use of symbol preceding at the DTV transmitter has no effect on the co-channel interference from analog television signals. At the DTV receiver, so long as the co-channel interference is not so large as to overload the receiver front-end and capture the system channel, it is advantageous to precede the data slicing circuitry with a comb filter for reducing the energy of higher-energy spectral components of the co-channel interference, thus to reduce the errors occurring during data slicing. The DTV broadcaster should adjust his carrier frequency, which is nominally 310 KHz above the lower limit frequency of the television channel assignment, so that his carrier frequency is optimally offset in frequency from the video carrier of a co-channel NTSC analog TV signal that is likely to interfere. This optimal offset in carrier frequency is exactly 59.75 times the horizontal scan line frequency fH of the NTSC analog TV signal. The artifacts of the co-channel interference in the demodulated DTV signal will then include beats at 59.75 times the horizontal scan line frequency fH of the NTSC analog TV signal, generated by heterodyne between the digital HDTV carrier and the video carrier of the co-channel interfering analog TV signal, and beats at 287.25 times fH, generated by heterodyne between the digital HDTV carrier and the chrominance subcarrier of the co-channel interfering analog TV signal, which beats are quite close in frequency to the fifth harmonic of the beats at 59.75 times fH. The artifacts will further include beats at approximately 345.75 times fH, generated by heterodyne between the digital HDTV carrier and the sound carrier of the co-channel interfering analog TV signal, which beats are quite close in frequency to the sixth harmonic of the beats at 59.75 times fH. The nearly harmonic relationship of these beats allows all these carrier frequencies to be suppressed using a single properly designed comb filter incorporating only a few symbol epochs of differential delay.
The inventor in U.S. Pat. No. 5,748,226 issued May 5, 1998 from application Ser. No. 08/746,520 filed Nov. 12 1996, entitled DIGITAL TELEVISION RECEIVER WITH ADAPTIVE FILTER CIRCUITRY FOR SUPPRESSING NTSC CO-CHANNEL INTERFERENCE and incorporated herein by reference describes how the modification of symbol coding that occurs when such comb filtering is used in the DTV signal receiver can be compensated for by postcoding that follows symbol decoding in the DTV receiver, rather than by preceding at the DTV transmitter. Data-slicing is a quantizing process that is not destructive of the symbols resulting from the comb filtering to suppress co-channel interfering analog TV signal, insofar as the transmission of data is concerned, since the data quantization levels are designed to match the symbol levels. The quantization discriminates against the co-channel interfering analog TV signal remnants that remain after the comb filtering and that are appreciably smaller than steps between symbol code levels, however. This is a species of the capture phenomenon in which phenomenon a stronger signal gains at the expense of a weaker one in a quantizing process. Insofar as the transmission of data is concerned, the digital data symbol stream flows through the full length of the system channel. When generalized symbol preceding of all data is done at the DTV transmitter, the additive combination of the differentially delayed data symbol streams is done on a modular basis that does not boost transmitter power or increase average intersymbol distance to help further in overcoming jamming analog TV signal. Instead, the principal mechanism for overcoming jamming analog TV signal is its attenuation vis-a-vis DTV signal, as provided by the comb filtering at the DTV receiver, causing the remnant analog TV signal in the comb filter response to be suppressed by the quantizing effects in the data slicer that immediately follows the comb filter. The order of performing the types of symbol re-coding associated normally with generalized symbol preceding of all data at the DTV transmitter and with comb filtering at the DTV receiver and has no appreciable affect on signal transmission through the system channel under such circumstances, since neither coding scheme destroys signal transmission capability for the symbol stream. The order of performing these symbol re-coding procedures has no appreciable affect on the capability of the digital receiver to suppress co-channel interfering analog TV signal, as long as the effects of the NTSC-rejection comb filtering are not undone before data-slicing. Attempts were made in the prior art to suppress the video carrier, the chroma subcarrier and the sound carrier of a co-channel interfering analog TV signal by using comb filtering before data-slicing within the DTV receiver. However, the effects of this comb filtering on the data-slicing procedures used for symbol decoding were presumed to be compensated for by preceding performed at the DTV transmitter, rather than postcoding being performed at the DTV receiver.
Comb filtering suppresses the sound carrier of the co-channel interfering analog TV signal successfully only when the frequency of that carrier is not modulated very much. The relatively high symbol rates (10.76 MHz) compared to the frequencies of the signal modulating the sound carrier help a comb filter that combines signals differentially delayed by only twelve symbols to suppress the sound carrier of the co-channel interfering analog TV signal reasonably adequately despite frequency modulation thereof. Suppression of the sound carrier of the co-channel interfering analog TV signal is not as good when the TV sound is strongly stereophonic or second audio program (SAP) is used. The types of strong correlation and anti-correlation evident in the video content of the co-channel interfering analog TV signal between closeby horizontal scan lines, between video frames, and between component fields of video frames do not obtain for frequency-modulated sound carrier, however. So, generally, comb filters that combine signals differentially delayed by periods of a video horizontal scan line or more do not suppress the sound carrier of the co-channel interfering analog TV signal very well.
A problem in analog TV receivers is the 920 kHz beat that appears in the response of the video detector owing to intermodulation between the chroma subcarrier and the sound carrier, unless the sound carrier is removed from the final IF signal. Such 920 kHz beat in the luminance signal causes visible artifacts in the analog TV visual display that are objectionable to most viewers, so the customary practice in analog TV receiver design is to suppress, xe2x80x9ctrap filterxe2x80x9d or xe2x80x9ctrapxe2x80x9d the sound carrier in the IF amplifiers. The inventor points out that, in the presence of a co-channel interfering analog TV signal, 920 kHz beat arises in the final IF signal of a DTV receiver as well, owing to intermodulation between the chroma subcarrier and the sound carrier. There are other beats, as well, owing to intermodulation between the video subcarrier and the sound carrier, and owing to intermodulation between the video subcarrier and the chroma subcarrier. These beats do not directly affect luminance in the DTV visual display, so as to suggest the use of in-channel sound traps in the DTV receiver. However, these beats do affect data slicing to slight degree.
A comb filter which differentially combines DTV baseband signals that are differentially delayed by twelve symbol epochs suppresses all these beat frequencies in some degree. Such comb filter is used for post-coding in a DTV receiver designed for use with a DTV transmitter using precoding in U.S. Pat. No. 5,260,793 issued Nov. 9, 1993 to R. W. Citta et alii and entitled RECEIVER POST CODER SELECTION CIRCUIT.
The inventor points out that filtering can be used in the IF amplifiers of DTV receivers for suppressing most of the energy in the frequency-modulated sound carrier of a co-channel interfering analog TV signal, while at the same time maintaining response over the band required for proper reception of the VSB DTV signal. The filtering does not need to employ a narrowband tracking filter that follows the FM sound carrier, either, which types of filter experience difficulty following deviation caused by SAP and professional channels. The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel, and the band required for the 10.76 MHz symbol rate signal extends to a frequency 5.38 MHz higher than that carrier frequency. The upper limit frequency of the band required for proper reception of the VSB DTV signal ends 5.69 MHz above the lower limit frequency of the TV channel. The sound carrier for an analog TV signal is 5.75 MHz from the lower limit frequency of the TV channel, which is to say 250 kHz below the upper limit frequency of the TV channel. If second audio program (SAP) and professional channels are included in the co-channel interfering analog TV signal, the frequency deviation of the audio carrier can be as high as 73 kHz or more. Filtering the IF response so that it cuts off the converted frequencies beyond those 5.69 MHz above the lower limit frequency of the TV channel will suppress most of the energy in the frequency-modulated sound carrier, however. The cut-off must be quite abrupt and should be accompanied by as little departure from phase linearity as possible.
DTV receivers are commonly plural-conversion receivers. In such a DTV receiver the data carrier is upconverted to an ultra-high-frequency (UHF) intermediate-frequency signal above the frequencies assigned as UHF television broadcasting channels and then amplified in a UHF intermediate-frequency amplifier. The response of the UHF IF amplifier is downconverted to a very-high-frequency (VHF) intermediate-frequency signal below the frequencies assigned as VHF television broadcasting channels and then amplified in a VHF intermediate-frequency amplifier. The VHF IF amplifier is usually a plural-stage amplifier, at least some of its stages being subject to automatic gain control (AGC). The IF filtering desiderata set forth in the preceding paragraph could be met by using a surface-acoustic-wave (SAW) filter operative in the VHF IF band to determine overall IF bandwidth. However, since a large number of zeroes and poles are required in order to get flat-amplitude linear-phase response in the passband together with steep-slope skirts, these IF filtering desiderata are more easily met using a SAW filter operative in the UHF IF band to determine overall IF bandwidth.
Whether the data slicing done during symbol decoding is preceded by comb filtering or is not, the reduction of the sound carrier and beats arising therefrom such SAW filtering provides will reduce the energy of spurious signal accompanying the symbol coding and so reduce the error in the data-slicing procedures employed during symbol decoding. If the data slicing done during symbol decoding is preceded by comb filtering, such SAW filtering allows the use of comb filters which suppress artifacts caused by video components of a co-channel interfering analog TV signal quite well but which either do not suppress artifacts of the audio components of that signal or do not suppress those artifacts very well. A comb filter that additively combines DTV baseband signals that are differentially delayed by six symbol epochs does not suppress artifacts of the audio components of a co-channel interfering analog TV signal very much, but suppresses artifacts caused by video components of the co-channel interfering analog TV signal quite well. The artifacts caused by video components of the co-channel interfering analog TV signal are usually better suppressed than the comb filter that subtractively combines DTV baseband signals that are differentially delayed by twelve symbol epochs, since anti-correlations at six-symbol intervals are more likely to be high than correlations at twelve-symbol intervals.
The invention is embodied in a digital television signal receiver with intermediate-frequency amplifier circuitry that includes a filter for suppressing the frequency-modulated sound carrier component of co-channel interfering analog television signal accompanying multiple-level symbols descriptive of data, to reduce error in the data-slicing procedures carried out on those multiple-level symbols during symbol decoding.
When a comb filter is used before data slicing to reduce the energy of co-channel interfering NTSC television signal, the filter in the IF amplifier circuitry for suppressing the frequency-modulated sound carrier component of co-channel interfering analog television signal eases the filtering requirements on the comb filter. Accordingly, a comb filter that suppresses principally video components of co-channel interfering NTSC television signal can be employed. Particularly, a comb filter that additively combines data differentially delayed by six symbol epochs can be employed for reducing the energy of the video components of co-channel interfering NTSC television signal.